Flick concedes Barca were disappointing for their 3-0 loss in London in midweek.
Barca meet Alaves this weekend and the German said: "When we lose, and when we concede three goals, it's easy to say it wasn't a good game and it wasn't the Barça we wanted to be, but we also have to make it clear that some people don't understand what we need to press high up the pitch.
"It's not just the defenders, it's the midfield and the forwards. Every player has to press. It seems like the defense is being blamed, and that's not fair. If we don't all press well, we'll have problems.
"I said I'm optimistic because we did some things well, but we have to improve, especially up front. I want to see what I see in training, which I like: the players have quality, they're focused, Pedri is back... Pedri, Raphinha, Marcus... they'll contribute.
"I want to see in matches what I see in training. And I tell the players what I see. And that's what I told you too. We're in a good moment and the feeling is good. We're going to work on it. We have to pick ourselves up after the defeat."
Asked about buying in January, Flick continued: "We're focused on what I have. On the match. We have just over three weeks left. There are a lot of matches to play and all my energy is focused on that."
On Raphinha's return, he added: "I've missed him. He's one of the most focused players on the team, he fits our style... he has a very positive impact on our game. He's hungry and eager to show how good he is, after a great season last year. I want and hope he shows how good he is."